Triazine-based porous organic polymers for reversible capture of iodine and utilization in antibacterial application

The capture and safe storage of radioactive iodine (129I or 131I) are of a compelling significance in the generation of nuclear energy and waste storage. Because of their physiochemical properties, Porous Organic Polymers (POPs) are considered to be one of the most sought classes of materials for iodine capture and storage. Herein, we report on the preparation and characterization of two triazine-based, nitrogen-rich, porous organic polymers, NRPOP-1 (SABET = 519 m2 g−1) and NRPOP-2 (SABET = 456 m2 g−1), by reacting 1,3,5-triazine-2,4,6-triamine or 1,4-bis-(2,4-diamino-1,3,5-triazine)-benzene with thieno[2,3-b]thiophene-2,5-dicarboxaldehyde, respectively, and their use in the capture of volatile iodine. NRPOP-1 and NRPOP-2 showed a high adsorption capacity of iodine vapor with an uptake of up to 317 wt % at 80 °C and 1 bar and adequate recyclability. The NRPOPs were also capable of removing up to 87% of iodine from 300 mg L−1 iodine-cyclohexane solution. Furthermore, the iodine-loaded polymers, I2@NRPOP-1 and I2@NRPOP-2, displayed good antibacterial activity against Micrococcus luteus (ML), Escherichia coli (EC), and Pseudomonas aeruginosa (PSA). The synergic functionality of these novel polymers makes them promising materials to the environment and public health.

metal-free condensation reactions. Previously, we reported on the synthesis of a number of nitrogen-rich aminallinked POPs (NRAPOPs) that displayed excellent iodine and CO 2 capturing capacities 24,25 . We attributed the high I 2 and CO 2 capture to the simultaneous effect of the high content of heteroatoms, porosity, and rich-π-electron conjugated structures, which contribute to the enhancement of the adsorbate-adsorbent interactions 26,27 . In this study, we extended the NRAPOPs library by preparing two cost-effective-novel porous polymers, NRPOP-1 and NRPOP-2, through metal free-Schiff base polycondensation reaction of 1,3,5-triazine-2,4,6-triamine (melamine) or 1,4-bis-(2,4-diamino-1,3,5-triazine)-benzene with thieno [2,3-b]thiophene-2,5-dicarboxaldehyde. The obtained porous polymers exhibit high iodine vapor capturing capacity of up to 317 wt% and have over 87% iodine removal efficiency from solution. The effects of heteroatoms and phenyl rings on the chemical and structural properties of the polymers and their contribution to iodine adsorption are discussed. Aside from their excellent iodine adsorption, the iodine-loaded NRPOPs (I 2 @NRPOPs) showed good antibacterial effectiveness against a wide range of bacteria that included Micrococcus luteus (ML), Escherichia coli (EC), and Pseudomonas aeruginosa (PSA) through their release of iodine. To the best of our knowledge, NRPOP-1 and NRPOP-2, present the first examples of porous organic polymers to utilize the encapsulated iodine in the antibacterial application.
Our study herein suggests that other classes of porous materials, such as metal organic frameworks (MOFs), would also be reliable adsorbents for the capture and reutilization of pollutants in the fields of public health and chemical industry.
Iodine adsorption and release studies. The gravimetric measurements based on iodine adsorption experiments were carried out by the following procedure: 40 mg of each polymer (NRPOP-1 and NRPOP-2) was taken in separate 10 mL glass vials. After noting the initial weight, the vials were placed in a glass jar container having an excess amount of solid iodine. The glass jar was then sealed and heated at 353 K and 1.0 bar in an oven. After the iodine adsorption, the system was allowed to cool down to room temperature and weighed afterward. The iodine adsorption capacities for the NRPOPs were calculated by the following equation: where m 1 and m 2 are masses of polymer before and after adsorption. The adsorption isotherm and capacity of the dissolved iodine were evaluated by placing 3 mg of fresh NRAPOPs in 0.3 g mL −1 iodine-cyclohexane solution. The progress of the iodine adsorption was checked by UV-Vis spectrometer. The iodine release experiments were carried out using ethanol as the solvent. For that, 50 mL of ethanol was added to a conical flask containing the 5 mg of the iodine-loaded NRPOPs, and the extent of iodine released was monitored by UV-Vis spectroscopy at specific time intervals.
Antibacterial activity of the iodine captured polymers. The antibacterial activity of the I 2 @NRPOPs was assessed against three bacteria species: Escherichia coli (EC, MicroKwik Culture # 155065A), Pseudomonas aeruginosa (PSA, MicroKwik Culture # 155250A), and Micrococcus luteus (ML, MicroKwik Culture # 155155A) using agar diffusion test. The bacterial inoculum was uniformly spread using a sterile cotton swab on a sterile Petri dish agar. Then, a sample (1 mg) of the iodine-loaded polymers was placed in the center of the dish and it was covered by a sterile wet (15 microL of water) paper disk (5 mm in diameter). For the control experiments, a sample of the unloaded polymer was used. After incubation for 24 h at 37 °C for EC and PSA, and at 25 °C for ML, the diameter of the growth inhibition zone was measured to determine the efficiency of the NRPOPs. Tests were performed in triplicates.
Instrumentation and methods. Surface area and pore size distribution of the synthesized material were measured by using nitrogen adsorption and desorption isotherm (Quantochrome analyzer at 77 K), which were calculated by using Braunauer-Emmett-Teller (BET) and the non-local density functional theory (NLDFT) methods, respectively. Fourier transform infrared spectra were recorded by Perkin-Elmer FT-IR spectrometer. Thermogravimetric analysis (TGA, Perkin-Elmer thermogravimetric analyzer) was carried out to check the thermal stability of the material at a heating rate of 10 °C min −1 in a temperature range of 30-800 °C under air atmosphere. Powder X-ray diffraction (PXRD) patterns were measured in 2θ range of 5-90° at a scanning speed www.nature.com/scientificreports/ of 5° min −1 in room temperature by using a Panalytical X'pert pro multipurpose diffractometer having a Cu Kα radiation. SEM images were recorded with high resolution scanning electron microscope (TESCAN-LMU SEM). Iodine solution concentrations were measured using a UV-Vis spectrometer (Shimadzu UV-1800 spectrometer) coupled with a quartz cuvette.
The successful formation of the NRPOPs was confirmed by FTIR spectroscopy (Fig. 1a). The aminal linkage formation between the triazine units and the aldehyde monomer is indicated by the appearance of stretching vibration bands at 3400/3380 and 2970/2965 cm −1 corresponding to secondary amine (N-H) and methylene (C-H) stretching. The inclusion of the triazine building block is confirmed by the presence of characteristic peaks at 1561/1552 and 1349/1352 cm −128, 29 . In addition, the disappearance of the primary amine's NH 2 (stretching/deformation) peaks and the aldehyde's stretching band of its C=O reveals the consumption of these groups which add another evidence for the formation of the NRPOPs (Fig. S1) 30,31 . The thermal stability of the as-prepared polymers was investigated by thermogravimetric analysis (TGA). Both NRPOPs show excellent  www.nature.com/scientificreports/ thermal stability up to about 350 °C (Fig. 1b) because of the strong covalent interactions between the building blocks. The initial weight loss (below 100 °C) is presumably due to the evaporation of the guest molecules such as acetone and water 32 . The morphology of the synthesized samples was examined by scanning electron microscopy (SEM). Images of NRPOP-1 and NRPOP-2 ( Fig. 2a,b) reveal that both polymers are composed of agglomerated spheres having different sizes. As the case of many POPs 24,25,33 , the two NRPOPs' PXRD patterns confirmed their amorphous nature by showing characteristic broad peaks (Fig. 2c).
Nitrogen adsorption-desorption experiments at 77 K were used to evaluate the porosity of the two POPs. As shown in Fig. 3a, the isotherms increase sharply at the lower pressure region (below 0.01 P/P 0 ) which is indicative of the microporosity nature of the NRPOPs 34 . The reversible adsorption-desorption isotherms with minimal hysteresis suggest that the two NRPOPs are rigid presumably due to the highly cross-linked structures ( Table 1 summarizes the textural properties of both polymers) 28,31 . The Brunauer-Emmet-Teller (BET) surface areas of NRPOP-1 and NRPOP-2 were determined to be 519 m 2 g −1 and 456 m 2 g −1 , respectively. The lower surface area and the smaller total pore volume (0.675 cm 3 g −1 ) of the NRPOP-2 polymer are presumably due to the existence of the phenyl group in the 1,4-bis-(2,4-diamino-1,3,5-triazine)benzene building block, leading to conformational flexibility in the polymer 3,33 . The pore size distribution (PSD) of the polymers were determined   www.nature.com/scientificreports/ using the nonlocal-density-functional theory (NLDFT). The highest peaks were observed at 1.2 (NRPOP-1) and 1.3 nm (NRPOP-2) as presented in Fig. 3b.
Iodine capture and adsorption of the NRPOPs. The iodine capturing capacity of the NRPOPs was tested by adding known amounts of the NRPOPs into glass vials and placing them into a sealed glass jar containing an excess amount of iodine crystals. The system was heated up to 350 K and 1 bar and the samples weights were measured after some time intervals. In the first 8 h, both materials exhibited a significant increase in iodine adsorption (~ 200 wt%) presumably due to the vast abundance of adsorbing sites with an apparent color change from yellow to black (Fig. 4 inset). Afterward, the loading capacity continued to increase but at slower rates and reached equilibrium after 72 h of contact time with maximum iodine uptakes of 269 and 317 wt% for NRPOP-1 and NRPOP-2, respectively (Fig. 4). Such capacities are either higher or comparable to other porous aminallinked porous polymers (Table S1). The obtained high iodine capacities of the two NRPOPs can be attributed to the presence of the electron deficient triazine moieties and -NH-unites which enhance the dipole-dipole interaction between the polymers structures and the iodine. In addition, the synergetic functionalities of the NRPOPs having both π-conjugated surfaces and rich-heteroatoms (N and S) provide another important favourable interaction between the adsorbates and the adsorbents 35,36 . The higher iodine uptake by NRPOP-2 can be attributed to the extra phenyl ring in its structure. Previous studies have shown that porous polymers bearing a plentiful number of phenyl rings enhance their affinity to adsorbates because they provide an electron-rich environment that enables stronger iodine adsorbent interactions 3,25,27,37 . The iodine loading capacities were also studied by the TGA of the iodine loaded (I 2 @NRPOPs). The I 2 -NRPOPs curves show steep weight loss in the temperature range of 90 to 350 °C (Fig. S2). Such weight loss is presumably due to the release of the encapsulated iodine since the pristine polymers are stable under this temperature range. The weight loss of the iodine was estimated to be 2.65 and 2.71 g g −1 for I 2 @NRPOP-1 and I 2 @NRPOP-2, respectively, which are close to the equilibrium adsorption values especially for that of NRPOP-1. The difference in the weight loss and the saturated value of the NRPOP-2 is probably due to the incomplete release of the captured iodine due to its stronger interaction with the polymer as reasoned above. Major peaks in the IR spectra of the I 2 @NRPOPs revealed a significant shift compared to NRPOPs (Fig. S3). For example, the secondary amine bands of the I 2 @NRPOP-1 and I 2 @NRPOP-2 are red shifted by 84 and 60 cm −1 , while the characteristic peaks of the triazine blue shifted by 49 and 66 cm −1 , respectively. Such observations clearly suggest that the iodine is being captured at the secondary amine, aromatic rings, and triazine units of the NRPOPs. Similar to these shifts in the IR spectra have been observed in the iodine loaded HCMP-3 and iodine-doped PANi, and were attributed to the formation of charge transfer (CT) complexes between the polymers and polyiodide ions 38,39 . PXRD measurements of the I 2 @NRPOPs showed only broad peaks indicating their amorphous nature and absence of pristine iodine crystals (Fig. S4) and hence suggesting the iodine is encapsulated inside the pores.
In addition to the gravimetric iodine vapor uptake, the ability of the synthesized polymers to adsorb iodine from the solution was investigated. In a typical experiment, 3 mg of the NRPOPs were immersed into 300 mg L −1 cyclohexane solutions of iodine (3 mL). With time, the color of the solutions gradually faded away from purple to light pink/colorless, indicating the removal of iodine from the solutions (Fig. 5a, inset). UV-Vis spectroscopic measurements were carried out to quantify the amount of the removed iodine and to monitor the progress of the adsorption rates. Figure 5a displays a plot of iodine percentage removal vs. time that clearly shows that the adsorption kinetics consists of two stages: (1) a rapid initial increase in the adsorptions in the first 2 h and (2) www.nature.com/scientificreports/ a slow steady increase afterward until equilibrium was reached. NRPOP-1 and NRPOP-2 exhibited an iodine removal efficiency of 59% (177 mg g −1 uptake) and 87% (261 mg g −1 uptake) from 300 mg L −1 cyclohexane solutions, respectively. The lower iodine uptake compared to that in the vapor phase is presumably due to the encapsulation of solvent molecules inside the NRPOPs pores. The faster and higher iodine removal efficiency of the NRPOP-2 can be related to the richer conjugated structure because of the presence of phenyl rings in its structure, and hence, leading to an enhanced iodine-host polymer interactions 3,40 . Recently, theoretical calculation (DFT) for iodine uptake studies have been reported that substantiate the experimental results 41 . The obtained kinetics results were fitted into pseudo-first and pseudo-second-order kinetic equations (SI). The calculated R 2 values (Table S2) indicate that the iodine adsorption kinetics fit better a pseudo-second-order kinetic model for both NRPOPs (Figs. 5b, S5). Such results suggest that the adsorption of iodine is governed by the chemisorption process 39,40 . This is attributed to the presence of high interaction between the iodine molecules and the surface of the NRPOPs. The presence of aromatic rings and electron-rich heteroatoms in the NRPOPs provide electron-donating moieties that are proficient in donating electrons to the electron-accepting iodine molecules, leading to charge transfer (CT) interactions and thus promoting higher iodine capturing capacity 3,24 . Previous reports indicated that such CT interactions are enhanced by the existence of -NH-units in the host structure 25,40 . This could be an additional factor for the excellent iodine uptake given the abundant -NH-containing pores of the NRPOPs.
To gain more insight into the adsorption mechanism of the iodine by the NRPOPs, we carried out adsorption equilibrium experiments by adding 1 mg of each NRPOPs into a range of 1 mL iodine solutions of various concentrations at 298 K. After 72 h of soaking, the residual concentrations of iodine in the solutions were determined by UV-Vis spectroscopy. The obtained isotherm data were then fitted into the two widely utilized isotherm models: Langmuir and Freundlich (Fig. 6a,b). The Langmuir model suggests a monolayer adsorption on the surface with homogenous distribution in the active sites, whereas the Freundlich model suggests a reversible  www.nature.com/scientificreports/ heterogeneous multilayer interaction 24,25 . The extracted isotherm parameters and linear regression coefficients are displayed in Table 2. The higher R 2 values for the Freundlich model indicate that the equilibrium adsorption isotherms can be well fitted using this model. Accordingly, the adsorption of the iodine by NRPOPs can be described as a heterogeneous multilayer reversible adsorption process. Such conclusion is supported by the presence of different iodine adsorption sites within the NRPOPs that include triazine, dithiophene, and -NH-units 3 .
Iodine release from iodine-loaded NRPOPs and potential applications. The encapsulated iodine can be readily released upon placing the I 2 @NRPOPs in polar organic solvents such as ethanol at room temperature. To evaluate the releasing process, 5 mg of the I 2 @NRPOPs were immersed in ethanol (50 mL) as time progressed, the color of the solution gradually changed from colourless to brown indicating the release of the encapsulated iodine (Fig. 7a inset). The amount of iodine released into the ethanol solution was monitored by UV-vis measurements (Fig. 7a) at different time intervals. I 2 @NRPOP-1 and I 2 @NRPOP-2 exhibited a linear and quick release of iodine in the first 2 min followed by slow but steady increase over almost 3 h to reach a maximum iodine release efficiency of 95.8 and 85.9%, respectively. The linear initial increase implies pseudozero-order kinetics which is inductive of host-guest interactions 37 . It is interesting to note that the slower and lower iodine release efficiency of I 2 @NRPOP-2 is consistent with the higher iodine capture because of the strong π-conjugation interactions which facilitate the iodine capture, but also reduce the release efficiency and prolong the release time.
Considering the efficient release of iodine by the loaded polymers, this feature of I 2 @NRPOPs was further investigated for two potential applications. For the first application, the release of iodine from the polymers allows for another cycle of iodine loading which enables the compounds to act as transport carriers to move iodine from one medium to another. For example, NRPOPs can be used to remove pollutant radioactive iodine from the environment and release it into controlled containers with subsequent reuse of the polymers for more cycles. For the second application, the iodine-loaded polymers can be used to deliver iodine into a specific environment with controlled quantities. For example, since iodine is known to have antiseptic properties effective against a broad range of both gram-positive and negative bacteria 42,43 , I 2 @NRPOPs can be used as a delivery vehicle of iodine in such areas as wound dressing or bandage to prevent bacterial infections.
Accordingly, we investigated the reusability of the polymers by first soaking samples of I 2 @NRPOPs in ethanol to release the iodine and then applied thermal activation to release solvent molecules from the pores of the polymers. The regenerated samples were then subjected to gravimetric iodine uptake experiments (as described above). The NRPOP-1 and NRPOP-2 were able to capture up to 119 and 168 g g −1 of iodine, respectively, which correspond to 44 and 53% retention of their initial iodine uptake capacity (Fig. 7b) after four loadingrelease-regeneration cycles. The decrease in the loading efficiency is probably due to the strong iodine-polymers Table 2. Langmuir and Freundlich parameters for the adsorption of Iodine. Langmiur model: Q m = maximum monolayer coverage capacity (mg g −1 ), K l = Langmuir isotherm constant (L mg −1 ); Freundlich model: K f = Freundlich isotherm constant (mg g −1 ), N = adsorption intensity; R 2 is the correlation coefficient of the linear fit.  S6 and S7). The results show the excellent stability and robustness of the POPs. The small shift toward the right side in the PXRD peaks after the fourth cycle is probably due to the incomplete release of the captured iodine by the stronger interaction between the polymer and iodine molecules.
Antibacterial activity of the captured iodine. In order to investigate the potential application of I 2 @ NRPOPs as a delivery vehicle of iodine for antiseptic action, we tested the ability of iodine-loaded NRPOPs to inhibit the growth of three types of bacteria, Escherichia coli (EC), Pseudomonas aeruginosa (PSA), and Micrococcus luteus (ML) using agar diffusion method. A sample (1 mg) of each iodine-loaded polymer was placed in the center of an agar petri dish seeded with the specific bacterial stain and the sample was covered by a wet paper disk. After incubation for 24 h, the diameter of the growth-inhibited zone was measured and compared to the control samples (polymers without iodine). The results presented in Table 3 ( Fig. S8) reveal that both iodine-loaded NRPOP-1 and NRPOP-2 inhibited the growth of gram-negative bacteria, PSA and EC, and grampositive bacteria ML with similar extents while the polymer alone (control) had no effect on bacterial growth. These results suggest that a significant amount of loaded iodine that was released from the polymer during the incubation time (24 h) prevented the growth of bacteria 44 , hence supporting the idea of potential application of NRPOPs as delivery vehicles of iodine for antiseptic applications.

Conclusion
In summary, we synthesized two nitrogen rich aminal-linked porous organic polymers, NRPOP-1 and NRPOP-2, by metal free-Schiff base polycondensation reaction of 1,3,5-triazine-2,4,6-triamine (melamine) and 1,4-bis-(2,4diamino-1,3,5-triazine)-benzene with thieno[2,3-b]thiophene-2,5-dicarboxaldehyde to attain the synergism of iodine uptake and the successive antibacterial use of the adsorbed iodine. Both polymers displayed excellent iodine vapor capturing capacity and were able to remove over 87% of dissolved iodine from 300 mg L −1 solution. The presence of phenyl units in the NRPOP-2 structure proved to be a major factor for the higher iodine uptake. The iodine-loaded polymers exhibited good antibacterial activity against the Micrococcus luteus (ML), Escherichia coli (EC), and Pseudomonas aeruginosa (PSA). The excellent iodine uptake and the effectiveness as antibacterial agents, NRPOPs make them promising candidates for environmental remediation and drug-delivery applications.